Device and method for controlling compression of tissue

ABSTRACT

A method and device for controlling the compression of tissue include clamping tissue between a first clamping member and a second clamping member by driving at least one of the clamping members with an electric motor toward a predetermined tissue gap between the clamping members and, during the clamping, monitoring a parameter of the electric motor indicative of a clamping force exerted to the tissue by the clamping members. The method and device include, during the clamping, controlling the electric motor, based on the monitored parameter, to limit the clamping force to a predetermined maximum limit.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation application of U.S. patentapplication Ser. No. 13/197,097, filed on Aug. 3, 2011 (now U.S. Pat.No. 8,210,413), which is a Divisional application of U.S. patentapplication Ser. No. 12/430,780, filed on Apr. 27, 2009 (now U.S. Pat.No. 8,012,170), the entire contents of all of which are incorporated byreference herein.

FIELD OF THE INVENTION

The present invention relates generally to medical devices. Morespecifically, the present invention relates to a device and method forcontrolled tissue compression.

BACKGROUND INFORMATION

Some surgical procedures require the compression, e.g., clamping, of apatient's tissue. Such procedures may include, e.g., anastomosing,stapling, and resecting of tissue. For example, where cancerous tissueis identified in a patient's gastrointestinal tract, the canceroustissue may need to be surgically removed. Where, for example, thecancerous tissue is located on the colon and is accessible by surgicalinstrumentation, the surgeon may make an incision in the patient'sabdomen to allow access to the bowel. The surgeon may then use a linearcutting and stapling device, such as that described in U.S. patentapplication Ser. No. 12/235,362 (now U.S. Pat. No. 7,963,433), which isexpressly incorporated herein in its entirety by reference thereto, tocut and staple the colon tissue on opposite sides of the cancerousportion to be removed. In this procedure, the colon is externallyclamped (e.g., between opposed jaws) to compress the tissue. While thetissue is compressed, a cutter and a stapler are activated to make alinear cut and apply typically two linear rows of staples in the areasadjacent the cut. The stapling thus closes both open ends of the portionof the bowel to be removed, as well as providing a temporary closure ofthe two cut ends of the bowel. This closure limits exposure of thesurrounding tissue to the interior of the bowel, thus limiting the riskof infection. After the cutting and stapling procedure, the cancerousportion of tissue may be removed from the patient's body.

After the resection of the cancerous tissue, the surgeon may employ ananastomosing and stapling device, e.g., a circular stapler/cutter, suchas that described in U.S. patent application Ser. No. 10/785,682 nowU.S. Pat. No. 7,342,983), which is expressly incorporated herein in itsentirety by reference thereto. During this procedure a head portion ispositioned within the colon adjacent one of the cut ends and a base orshaft portion is positioned within the colon adjacent the other cut end.The head portion and the base portion may be coupled via a shaft and/orcable that extends out of one cut end and into the other. Via thiscoupling, the surgeon is able to actuate the anastomosing and staplingdevice to draw the head portion and the base portion together. After thetwo cut ends of the colon contact each other, the actuation continuessuch that the two portions of the colon are clamped together at anannular area of contact. While clamped, the anastomosing and staplingdevice may be further actuated to apply an annular ring of staples intothe compressed tissue. The device may also cut excess tissue disposedwithin the colon. The head portion and the base portion are then movedapart and the anastomosing and stapling device removed from the patient.

To achieve effective stapling in the above procedures, the tissue mustbe compressed to the extent that there is an adequately small tissuegap, e.g., one millimeter, between the faces of the tool. If theclamping structures of the instrument are exposed to enough force,maintaining a uniform target tissue gap across the length of tissue tobe stapled may be difficult or even impossible. For example, where theclamping structures are cantilevered jaws of a linear stapler, the jawsmay splay outwardly from each other under high clamping forces. Whereone or both of the jaws splay in this manner, the tissue gap typicallyincreases toward the distal ends of the jaws. Where this tissue gapexceeds an acceptable range, staples may not adequately close the tissueto prevent contamination. This may be result from, e.g., the initialstapled gap being too large and/or failure of the staple (e.g.,separation from one or more of the portions of stapled tissue) due toimproper formation resulting from, e.g., too large a gap between astaple pusher and an anvil that closes the staple.

Such problems with the stapling procedure may lead to contamination oftissue (e.g., contamination of tissue adjacent the bowel with bowelcontents), which may contribute to infection and/or sepsis. Suchproblems with the stapling procedure may also lead to, e.g., failure ofthe anastomosis (e.g., where the stapled tissues separate) and/orexcessive bleeding due to improper tissue closure. Moreover, theseproblems may require additional, repeated, and/or prolonged surgeryalong with any increased risks associated therewith. As reported by theUnited States Food and Drug Administration (see “Surgical StaplerInformation,” “Other Data,” athttp://www.fda.gov/cdrh/surgicalstapler/other_data.html, last updatedJul. 21, 2004), infection, sepsis, anastomosis failure, and bleeding aresubstantial problems that arise in stapling procedures and maypotentially lead to serious injuries, or even death, to some patients.It is thus desirable to minimize these problems.

Moreover, when performing the compression, a constant closing rate(e.g., the closing rate between jaws of a linear stapler or between thehead and base portion of a circular stapler/cutter) may exert a highlevel of power into the clamped tissue. This high level of power mayresult in excess tissue trauma. It is thus desirable to limit thistrauma, e.g., by effectively controlling the power applied to thetissue. Further, it is desirable to determine whether the tissue to beclamped is compressible.

U.S. Patent Application Publication No. 2009/0057369 (now U.S. Pat. No.7,959,050) describes a device that uses continuous measurements from alinear force switch housed in an anvil neck. The switch is calibrated toactivate when a given load is applied. The given load is set tocorrespond to a desired pressure that is to be applied to the particulartissue before stapling can occur. Interfacing this switch with aprocessor provides firing of staples only within a compression range.Such devices and control methods do not allow for a continuous closureor monitoring of power going into the compressed tissue.

Further, it is desirable to monitor and track structural fatigue inclamping members in a simple and reliable manner.

It is additionally desirable to identify proper staple filing in asimple and reliable manner.

SUMMARY

Example embodiments of the present invention provide a device and methodfor controlling compression of tissue.

According to an example embodiment of the present invention, a methodincludes clamping tissue between a first clamping member and a secondclamping member by driving at least one of the clamping members with anelectric motor toward a predetermined tissue gap between the clampingmembers. The method also includes, during the clamping, monitoring aparameter of the electric motor indicative of a clamping force exertedto the tissue by the clamping members. The method also includes, duringthe clamping, controlling the electric motor, based on the monitoredparameter, to limit the clamping force to a predetermined maximum limit.

The predetermined maximum limit may be selected based on predeterminedproperties of the tissue to be clamped.

The predetermined maximum limit may be selected to limit trauma to theclamped tissue to a predetermined acceptable level.

The predetermined maximum limit may be below a predetermined deflectionlimit of the at least one of the clamping members.

The controlling may include adjusting a voltage applied to the electricmotor. The controlling may include limiting the current driving theelectric motor, e.g., to a predetermined maximum value.

The monitoring may include measuring a current driving the electricmotor.

The method may include, prior to the clamping, measuring current due tofrictional losses associated with moving the clamping members andsubtracting the frictional losses from the current measured during theclamping.

The method may include, prior to the clamping, measuring a currentoffset.

The method may include subtracting the current offset from the currentmeasured during the clamping.

The predetermined maximum limit may be selected to prevent the forcefrom reaching a yield force.

The predetermined maximum limit may be selected to prevent the clampingmembers from splaying.

The method may include using a profile of the monitored parameter tomonitor and track structural fatigue in at least one of the clampingmembers. The profile may be compared to a normal parameter signature.

The surgical instrument may be a surgical stapler and one of theclamping members may be an anvil arranged to form surgical staples.

The method may include using a profile of the monitored parameter toidentify whether all of the staples of a staple cartridge have fired.

The method may include using a profile of the monitored parameter toidentify at least one of staple misfires, an absence of staples in astaple cartridge, and staple jams.

The controlling may include, e.g., determining a motor velocity,determining a motor position, and determining a net motor currentattributable to the compression by subtracting out a current offset andcurrent due to frictional losses. The controlling may also includedetermining a velocity drive based on the determined velocity,determining a position drive based on the determined position, anddetermining a current drive based on the determined net motor current.The controlling may also include applying the smallest of the velocitydrive, the position drive, and the current drive to the motor.

The method may include detecting the presence of an incompressibleobject based on the motor velocity.

The controlling may include adjusting a driving speed of the motor tolimit the clamping force to the predetermined maximum limit.

According to an example embodiment of the present invention, a surgicaldevice includes a first clamping member, a second clamping member, andan electric motor configured to drive at least one of the first andsecond clamping members toward a predetermined tissue gap between theclamping members. The device also includes a control system to monitor aparameter of the electric motor indicative of a clamping force exertedto the tissue by the clamping members and to control the electric motor,based on the monitored parameter, to limit the clamping force to apredetermined maximum limit.

The predetermined maximum limit may be selected based on predeterminedproperties of the tissue to be clamped.

The predetermined maximum limit may be selected to limit trauma to theclamped tissue to a predetermined acceptable level.

The predetermined maximum limit may be below a predetermined deflectionlimit of the at least one of the clamping members.

The surgical device may be arranged as at least one of (a) a linearsurgical stapler, (b) a circular surgical stapler, and (c) a right-anglelinear cutter. It should be appreciated that the surgical device may bearranged as a surgical device including clamping components, includingstaplers, ligators, etc.

The surgical device may include a current sensor to determine a currentdriving the electric motor, a velocity sensor to determine therotational velocity of the electric motor, and a position sensor todetermine at least one of the position of an output of the motor and arelative position of the first and second clamping members.

The control unit may be adapted to control the electric motor byadjustment of a driving speed of the motor to limit the clamping forceto the predetermined maximum limit.

According to an example embodiment of the present invention, a surgicaldevice is configured to monitor a parameter of an electric motorindicative of a clamping force exerted on tissue being clamped, controlthe electric motor to determine if a predetermined maximum limit isreached for the clamping force and limit the parameter of the electricmotor, and monitor the clamping force to determine variations in theclamping force and allow for the electric motor to reactivate once theclamping force exerted on the tissue has fallen below the predeterminedmaximum limit.

According to an example embodiment of the present invention, a surgicaldevice is configured to control a compressive force of a first clampingmember and a second clamping member toward a predetermined tissue gapand, once said tissue gap is attained, deploy at least one of tissuefasteners, energy, and adhesive fluids so that said tissue will remainapproximated during a healing process.

According to an example embodiment of the present invention, a surgicaldevice includes a clamping mechanism configured to clamp tissue, anelectric motor configured to drive the clamping mechanism, and a controlsystem. The control system is configured to: monitor a parameter of theelectric motor indicative of a clamping force exerted on tissue beingclamped; control the electric motor to determine if a predeterminedmaximum limit is reached for the clamping force and limit the parameterof the electric motor, and monitor the clamping force to determinevariations in the clamping force and allow for the electric motor toreactivate once the clamping force has fallen below the predeterminedmaximum limit.

According to an example embodiment of the present invention, a surgicaldevice includes a first clamping member, a second clamping member, and acontrol system. The control system is configured to: control acompressive force of the first and second clamping members toward apredetermined tissue gap between the first and second clamping members;and, once the tissue gap is attained, deploy at least one of tissuefasteners, energy, and adhesive fluids so that the tissue will remainapproximated during a healing process.

An example method for controlling compression of tissue in accordancewith the present invention includes positioning tissue in a tissuecompression mechanism, e.g., the jaws of a linear cutting and staplingdevice or the anvil and base of a circular cutting and stapling device.The method also includes compressing the tissue according to a closingrate.

The method further includes determining the instantaneous energy, orpower, and/or accumulated energy, or power, applied to the tissue duringcompression. The method further includes reducing the closing rate whenthe imparted power increases, e.g., to a predetermined level. Themeasuring may include measuring the current applied to a direct currentmotor. The current may be measured continually, e.g., at a fixedinterval, over the course of the compression. The closure rate may beadjusted so that the power imparted into the tissue remains constantover a portion of the compression.

An example device for controlling compression of tissue in accordancewith the present invention may include a clamping or compressionmechanism. The compression mechanism may be any appropriate mechanism,e.g., rotating or parallel jaws, an anvil and base of a circularstapling and cutting mechanism, and the like. The compression mechanismis actuated by an actuator which may include one or more electric motorswhich may be disposed in a remote console and/or the housing of thedevice. Power may be transferred from the actuator to the compressionmechanism by any appropriate structure, e.g., one or more rotatingshafts arranged to transmit rotational force. Where the actuatorincludes a direct current motor, the control system, e.g., a digitaland/or analog control system, may control the current going into theactuator by altering the voltage applied thereto. The control system maybe structured and/or programmed to limit the current going into themotor. The control system may be programmed to calculate the powerapplied to the tissue based on the measured current going into the motoras indicated by a current sensor. The control system may be programmedto calculate the rate of closure of the compression mechanism based onthe voltage input.

Moreover, the controller may be programmed to reduce the rate of closureby altering, e.g., lowering, the voltage input into the actuator, inorder to limit the power going into the tissue and/or maintain the powergoing into the tissue at a constant level.

Further details and aspects of example embodiments of the presentinvention are described in more detail below with reference to theappended figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph illustrating the power applied to tissue duringcompression at a constant anvil closing rate.

FIG. 2 is a graph illustrating the power applied to tissue duringcompression according to an example embodiment of the present invention.

FIG. 3 is a superposition of the graphs of FIG. 1 and FIG. 2.

FIG. 4 is a schematic illustration of a control system according to anexample embodiment of the present invention.

FIG. 5 is a flowchart illustrating a method according to an exampleembodiment of the present invention.

FIGS. 6 a and 6 b schematically illustrate a splaying effect of jaws ofa stapling device.

FIGS. 7 a to 7 d illustrate current profiles associated with differentstapling events.

DETAILED DESCRIPTION

During compression of a patient's tissue, hydraulic effects are presentas a result of the composition (e.g., the presence of fluids, etc.) ofthe tissue. In this regard, hydraulic resistance may be measured andused as feedback for the closing of the clamping elements.

When clamping the patient's tissue, forces exerted through the clampingdevice, e.g., a linear stapler, and the tissue may reach an unacceptablyhigh level. For example, when a constant closure rate is employed, theforce may become high enough to cause excess trauma to the clampedtissue and may cause deformation in the clamping device such that anacceptable tissue gap is not maintained across the stapling path. Forexample, an acceptable tissue gap may be in the range of, e.g., 1 mm±0.4mm,

$1\mspace{14mu}{mm}\frac{+ 0.4}{- 0.3}\;{mm}$(0.7 mm to 1.4 mm), etc. Referring to FIGS. 6 a and 6 b, linear surgicalstaplers 1000 and 1100 are schematically illustrated when exerting aclamping force on a section of tissue.

As illustrated, the high level of force exerted by the jaws of thestaplers 1000 and 1100 results in a splaying effect, which has beenexaggerated in FIGS. 6 a and 6 b for illustration purposes. Referring toFIG. 6 a, a pair of opposed jaws 1005 and 1010 form a target tissue gap1020 only at a proximal portion, whereas the distal ends of the jaws1005 and 1010 are splayed outwardly away from each other, resulting inan expanded tissue gap 1025 at a distal portion. This splaying causesthe jaws 1005 and 1010 to deviate from a parallel alignment, which maylead to an unacceptably large tissue gap where staples are appliedtoward the distal ends of the jaws 1005 and 1010, which in turn mayresult in the aforementioned difficulties, such as, e.g., leakage,contamination, and failed staple connections. This splaying results fromexceeding a yield force that causes the jaws to deviate from theparallel alignment relative to each other.

FIG. 6 b shows a comparable effect, but where the splaying occursprimarily in a first jaw 1105 that is structurally weaker than a secondjaw 1110. This again leads to an unacceptably large tissue gap 1125 thatsubstantially exceeds a target tissue gap 1120.

FIG. 1 is a graph illustrating the power applied to tissue duringcompression at a constant anvil closing rate. The compression begins atan initial open state, wherein the clamping members or elements move adistance prior to compression of the tissue therebetween. Region Arepresents the time during which the clamping elements move from theinitial open state to the beginning of tissue compression and region Brepresents the time period during which the tissue is compressed from aninitial thickness to a target thickness, in this case 1 mm tissuethickness (corresponding to a 1 mm tissue gap between the clampingelements). Regions A′ and B′ are delineated by a “knee” that indicatesthe beginning of the compression with the tissue at its initialthickness. The power, and in turn the force, imparted into the tissuerises sharply with respect to time until reaching a peak value. Line 10indicates the closing rate of the clamping members (indicated in FIG. 1as Anvil Closing Rate), which is constant between the initial open stateand the target tissue gap of 1 mm. The hatched area under the powercurve indicates the total energy exerted during the clamping procedure.

FIG. 2 is a graph illustrating the power applied to tissue duringcompression according to an example embodiment of the present invention.As with the device and method corresponding to the graph of FIG. 1, thecompression as illustrated in FIG. 2 begins at an initial open state, inwhich the clamping members or elements move a distance prior tocompression of the tissue therebetween. Region A′ represents the timeduring which the clamping elements move from the initial open state tothe beginning of tissue compression, and region B′ represents the timeperiod during which the tissue is compressed from an initial thicknessto a target thickness, in this case 1 mm tissue thickness (correspondingto a 1 mm tissue gap between the clamping elements). A′ and W aredelineated by a “knee” that indicates the beginning of the compressionwith the tissue at its initial thickness. During the period designatedA′, the clamping elements close at a constant rate of, e.g., 13 mm/sec.It should be appreciated, however, that any appropriate rate may beemployed and need not be constant over the entire period A′. The hatchedarea under the power curve indicates the total energy exerted during theclamping procedure.

In contrast to FIG. 1, FIG. 2 illustrates that it is determined that thepower applied to the tissue is increasing and, at a certain level, theclosing rate is decreased, in this example, from 13 mm/sec to 1 mm/sec,effectively increasing the time required to compress the tissue anddecreasing the power applied to the tissue. The closing rate isillustrated in FIG. 2 as line 100. In this example, the power applied tothe tissue is held constant, although it should be appreciated thataccording to certain example embodiments, the power may fluctuate.

FIG. 3 is a superposition of the graphs of FIG. 1 and FIG. 2. Incontrast to the system and method reflected in FIG. 1, the peak powerimparted into the tissue according to FIG. 2 is much lower. Based on theimparted power, the force exerted by the surgical device (or a parameterrelated to or proportional to the force) may be calculated. In thisregard, the power may be limited such that the force exerted through thesurgical device, e.g., through the jaws of a linear stapler, do notexceed a yield force or pressure that results in splaying of the jawssuch that the tissue gap is not within an acceptable range along theentire stapling length when in the fully closed position. For example,the jaws should be parallel or close enough to parallel that the tissuegap remains within the acceptable or target range for all staplepositions along the entire length of the jaws. Further, the limitationof the exerted power avoids, or at least minimizes, trauma or damage totissue.

In this example, the total energy exerted in the method of FIG. 1 is thesame as the total energy exerted in the method of FIG. 2, i.e., theareas under the power curves of FIGS. 1 and 2 are the same orsubstantially the same. The difference in the power profiles utilizedis, however, substantial, as the peak power is much lower in the exampleof FIG. 2 as compared to FIG. 1.

The limiting of power is achieved in the example of FIG. 2 by slowingthe closing rate, as illustrated by line 100. It is noted that thecompression time B′ is longer than the closing time B. As illustrated inFIGS. 1 to 3, a device and method that provides a constant closure rateachieves the same 50 lb of compressive force at the same 1 mm tissue gapas the device and method reflected in connection with FIG. 2. While thedevice and method that provide for a constant closure rate (FIG. 1) mayachieve the compressive force at the desired tissue gap in a shortertime period as compared with FIG. 2, as illustrated, e.g., in FIG. 1,this results in the spike in power applied to the tissue. In contrast,the example embodiment illustrated, e.g., in connection with FIG. 2,begins slowing the rate of closure to limit the amount of power appliedto the tissue below a certain level. By limiting the power applied tothe tissue, tissue trauma may be minimized with respect to the systemand method reflected in FIG. 1.

According to example embodiments, the device and method may beimplemented by determining the power or force applied to the tissue bymeasurement of the current applied to an actuator, since the current isproportional to the torque output of the motor. In this regard, lossesbased on the instrument, e.g., due to friction between moving parts,etc., may be subtracted from the power applied to the driving motor tomore accurately determine the power that is being imparted into thetissue. These losses to be backed out may be determined in anyappropriate manner such as, e.g., testing the instrument or componentsof the instrument, using known qualities of the instrument or componentsof the instrument, and/or performing calculations based on the testingand/or known qualities. For example, the instrument may be driven in anunloaded condition to obtain a baseline measurement of power or currentrequired to drive the instrument and its associated components.Thereafter, power or current in excess of the baseline corresponds tothe power that is applied to the tissue during compression.

Where, for example, the actuator is a direct-current electric motor, thepower applied to the motor may be determined based on a measurement ofthe current required to drive the motor. The losses due to theinstrument are then backed out to determine the power imparted into thetissue during the compression. This measurement allows a source offeedback when compressing the tissue. The power applied to the motor maybe continually monitored with calculations being performed on acontinuous basis. This allows, for example, the power or force appliedto the tissue to be accurately controlled, e.g., by adjusting thevoltage going into the motor. In this example feedback control system,the consumed current would be the feedback, with the voltage beingadjusted to achieve a desired current. The example illustrated in thegraph of FIG. 2 uses this control system to reduce the rate of closurewhen the determined power hits a particular level, e.g., a predeterminedpower level selected to prevent unacceptable levels of tissue trauma.

FIG. 4 is a schematic illustration of a control system according to anexample embodiment of the present invention. A controller 400 controls amotor 410 that drives a clamping operation, e.g., clamping of the jawsof a linear surgical stapler. A target position, a target current, and atarget velocity are input into the controller 400, e.g., by a particularcontrol program and/or manual input by a surgeon or operator. Thecontroller 400 receives motor position, motor velocity, and motorcurrent signals as feedback for controlling the motor 410. As discussedin greater detail below, the controller 400 according to this exampleselects the smallest input for controlling the motor 410.

FIG. 5 is a flowchart illustrating a method according to an exampleembodiment of the present invention. System startup occurs at 500. Aftersystem startup, current offset is measured and set to be subtracted fromall current readings so that all readings are taken from a zero ornear-zero baseline. At 510, calibration occurs, including measuringcurrent due to frictional losses, e.g., friction in the motor and thedrive components that convert the rotational force of the motor into theclamping force exerted through the instrument, e.g., the force exertedthrough the jaws of a linear stapler. The calibration may be performedby measuring the current corresponding to different motor speeds under ano-load condition, i.e., during unobstructed movement. It should beappreciated that the offset measurement and calibration may be performedevery time the system starts up and/or the values obtained may be storedto be used in subsequent procedures using the same equipment. Forexample, the control system may require re-measuring of these valuesafter a given time period, number of uses, and/or number of systemstartups.

After calibration, the clamping procedure may begin. When the tissue tobe clamped is disposed in the clamping portion of the surgicalinstrument, movement is started at 515. The exemplary method thenperforms a current loop, a velocity loop, and a position loop. Theseloops need not be performed in any particular order and two or all threeof the loops may be performed simultaneously, or substantiallysimultaneously, in some examples.

For the current loop at 600, motor current is read, e.g., according tothe signal of a current sensor arranged to sense the current driving themotor. The offset and frictional losses, determined at 505 and 510, areremoved or subtracted out. In this manner, the portion of the currentthat is applied in response to the tissue clamping is determined. At605, a current drive is calculated using a current drive formula. Forexample, the current drive may be determined by K₁*(target current−motorcurrent), where K₁ is selected based on desired control performance forcontrolling the motor current.

For the velocity loop at 700, the motor velocity is determined. Thevelocity is determined either by reading a signal from a velocity signalor any other appropriate manner, e.g., from position and time data. At705, a velocity drive is calculated using a velocity drive formula. Forexample, the velocity drive may be determined by K₂*(targetvelocity−motor velocity), where K₂ is selected based on desired controlperformance for controlling the motor velocity. At 710, it is determinedwhether an immovable object has been reached. In this regard, a velocityvalue of zero is indicative of an immovable object or obstruction beingreached by the clamping device. If an obstruction has been reached, thedriving of the motor is then stopped at A. Otherwise, the controlcontinues. It should be appreciated that this determination may be madebefore, after, and/or at the same time as the calculation of thevelocity drive.

For the position loop at 800, the motor position is read. The motorposition may be determined, e.g., by an encoder or a resolver coupled toan output of the motor, or any other appropriate manner. At 805, aposition drive is calculated using a position drive formula. Forexample, the position drive may be determined by K₃*(targetposition−motor position), where K₃ is selected based on desired controlperformance for controlling the motor position.

At 810, it is determined whether a target position has been reached. Ifthe target position has been reached, the control loop exits at B. At B,the output to the motor may be stopped (e.g., where the tissue isclamped using drivers that are not back-drivable by residual pressuresexerted by the clamped tissue or by the force of a staple being drivenand formed between the clamping members) and/or the motor may becontrolled to output an amount of force needed to maintain the motor atthe target position, which generally corresponds to the target tissuegap in the examples described above. If the target position has not beenreached, the control continues. It should be appreciated that thedetermination of whether the target position has been reached may bemade before, after, and/or at the same time as the calculation of theposition drive. Further, it should be appreciated that the relativeposition of the clamping elements, e.g., jaws, or any intermediatecomponent, e.g., a driver, may be used as a positional input.

After the three control loops, the calculated current drive, velocitydrive, and position drive, are compared, and the smallest drive isapplied to the motor at 900. At 905, the applied torque, which isproportional to and determined from the motor current after subtractingout the offset and frictional losses, is reported. At 910, the appliedtorque is accumulated to calculate the energy applied to the tissue.

Chart 950 illustrates control prioritization for three differentsituations. In the first situation, the motor velocity and motorposition are below their respective targets, while the motor position isnot. In this situation, the position loop controls, while the velocityand current loops are set to maximum values. In the second situation,the velocity and position loops are below their respective targets,while the current loop is not. In this situation, the current loopcontrols the output, while the velocity and position loops are set tomaximum values. In the third situation, the current and position loopsare below their respective targets, while the velocity is not. In thissituation, the velocity loop controls, while the current and positionloops are at maximum values.

It is determined at 915 whether an excessive time is required to attainthe target position. This determination may be made by, e.g., examiningthe amount of time that has elapsed up until the decision 915, apredicted total amount of time based on the elapsed time and the controlprofile (e.g., current, velocity, and position), and/or any otherappropriate manner. If it is determined that the required time isexcessive, the exemplary control method exits at C. At C, the controloutput to the motor may stop or another control method may be employed,e.g., to reverse the position of the motor. For example, the motor maybe driven to move the jaws of a linear stapler to an open position sothat the surgeon or operator may remove the surgical device or move thejaws to a different portion of tissue that may be easier to clamp. Inother words, at C a request or requirement user intervention may betriggered.

If it is determined that an excessive time is not required, the controlsystem again executes the current, velocity, and position loops at 600,700, and 800, respectively. This loop continues until one of the eventsA, B, and/or C occurs to break the loop. It should be appreciated,however, that additional conditions may be implemented to break theloop, e.g., a manual override, a sensor error, etc.

As indicated above, after the target position is reached at B, the motormay be controlled to maintain a force necessary to maintain the targetposition. The current driving the motor may be monitored at this stagefor a variety of purposes. For example, where the device is, e.g., asurgical stapler, a profile of the measured current may be used toidentify whether all of the staples of a staple cartridge have fired.FIG. 7 a illustrates an expected current profile during a staplingprocedure as a driver, e.g., a wedge, sequentially drives five staples.It should be understood that any number of staples may be provided andthe firing of five staples is for illustration purposes. The peaks inthe current measurement correspond to the increased force or powernecessary to hold the tissue gap with the staple be forced between theclamping elements, e.g., the jaws of a linear stapler. If the firing isinitiated and a current profile that closely resembles FIG. 7 a results,it may be determined that all of the staples properly driven or fired.

If the staple driving procedure is initiated and results in a currentprofile as illustrated in FIG. 7 b, it may be determined from the lackof a peak that the second staple position was not properly driven orfired, e.g., due to a misfire or a missing staple from the staplecartridge. A misfire may similarly be shown, e.g., when a current peakis present, but substantially lower than expected.

If the staple driving procedure is initiated and results in a currentprofile as illustrated in FIG. 7 c, it may be determined that the staplein the third staple position jammed in some manner, resulting in thehigher peak current measurement.

If the staple driving procedure is initiated and results in a currentprofile as illustrated in FIG. 7 d, it may be determined that the staplecartridge is empty or that a staple cartridge is not present.

If any of these unexpected events occur, the control system may alertthe user, e.g., by emitting an audible alarm and/or displaying an errormessage on a computer screen. The control system also may abort thestaple firing, and or enter a different control algorithm.

Moreover, a profile of the current measurements may be used to monitorand track structural fatigue in at least one of the clamping members,e.g., by comparing to a normal current or motor signature. In thisregard, a current profile may indicate, e.g., flexure due to plasticdeformation resulting from fatigue failure. Further, the current profilemay be used to track accumulated fatigue by determining the amount offorce and the number of cycles exerted by a clamping member.

It should be appreciated that example methods according to the presentinvention may be implemented using any appropriate control system, e.g.,a digital and/or analog control system, which may be integrated into themedical device or may be remotely located, whereby control and feedbacksignals are communicated via, e.g., a wireless or wired interface. Thecontrol system may have a display output, e.g., a monitor, and/or inputsto communicate with, e.g., a surgeon. The display output may displaydata relevant to the procedure including, e.g., the current closingrate, compressive force, and/or tissue gap. The control system may runpredefined control programs or algorithms that may be pre-selected forthe particular device. The control system may additionally oralternatively ask for inputs from the operator to define the parametersof the tissue compression control.

Further, the compressibility of the tissue may be determined byexamining the current applied to the motor as compared with the closingrate. For example, if the measured current is very high when using a lowclosing rate, the tissue is less compressible than situations where thecurrent is low for a higher closing rate.

While a tissue gap of 1 mm is mentioned above as an example of a desiredtissue gap appropriate for tissue stapling, it should be appreciatedthat instead of an absolute distance measurement for the gap,alternative gap parameters may be provided. For example, one or moreoptical sensors may be provided to measure blood flow across one or morestaple lines as a measure of desired tissue gap. Furthermore, oxygensaturation may be used in connection with the determination of thedesired tissue gap. Moreover, the ratio of compressed to uncompressedtissue, e.g., based on the power applied to the tissue at the knee inthe graphs of FIGS. 1 and 2, may form the basis of the desired tissuegap.

What is claimed is:
 1. A surgical device, comprising: a first clampingmember; a second clamping member; an electric motor configured to driveat least one of the first and second clamping members toward apredetermined tissue gap between the clamping members; and a controlunit configured to monitor the electric motor and to control the motorto limit a clamping force exerted on the tissue by the clamping membersin response to monitoring of the electric motor.
 2. The surgical deviceof claim 1, wherein the control unit is configured to prevent operationof the motor beyond a predetermined maximum limit based on predeterminedproperties of the tissue to be clamped.
 3. The surgical device of claim2, wherein the predetermined maximum limit is selected to limit traumato the clamped tissue to a predetermined acceptable level.
 4. Thesurgical device of claim 2, wherein the predetermined maximum limit isbelow a predetermined deflection limit of the at least one of theclamping members.
 5. The surgical device of claim 1, wherein thesurgical device is at least one of: (a) a linear surgical stapler, (b) acircular surgical stapler, or (c) a right-angle linear cutter.
 6. Thesurgical device of claim 1, wherein the control unit comprises: acurrent sensor configured to determine a current driving the electricmotor; a velocity sensor configured to determine a rotational velocityof the electric motor; and a position sensor configured to determine atleast one of a position of an output of the electric motor and arelative position of the first and second clamping members.
 7. Thesurgical device of claim 1, wherein the control unit is adapted tocontrol the electric motor by adjustment of a driving speed of theelectric motor to limit the clamping force.
 8. A surgical device,comprising: a clamping mechanism configured to clamp tissue; an electricmotor configured to drive the clamping mechanism; and a control unitconfigured to monitor a parameter of the electric motor indicative of aclamping force exerted to the tissue by the clamping mechanism and tocontrol the electric motor, based on the monitored parameter, to limitthe clamping force.
 9. The surgical device of claim 8, wherein thecontrol unit is configured to prevent operation of the electric motorbeyond a predetermined maximum limit based on predetermined propertiesof the tissue to be clamped.
 10. The surgical device of claim 9, whereinthe predetermined maximum limit is selected to limit trauma to theclamped tissue to a predetermined acceptable level.
 11. The surgicaldevice of claim 9, wherein the predetermined maximum limit is below apredetermined deflection limit of the clamping mechanism.
 12. Thesurgical device of claim 8, wherein the surgical device is at least oneof: (a) a linear surgical stapler, (b) a circular surgical stapler, or(c) a right-angle linear cutter.
 13. The surgical device of claim 8,wherein the control unit comprises: a current sensor configured todetermine a current driving the electric motor; a velocity sensorconfigured to determine a rotational velocity of the electric motor; anda position sensor configured to determine at least one of a position ofan output of the electric motor and a relative position of the clampingmechanism.
 14. The surgical device of claim 8, wherein the control unitis adapted to control the electric motor by adjustment of a drivingspeed of the electric motor to limit the clamping force.
 15. A methodfor controlling a surgical device, the method comprising the steps of:energizing an electric motor coupled to a clamping mechanism to drivethe clamping mechanism to clamp tissue; monitoring a parameter of theelectric motor indicative of a clamping force exerted to the tissue bythe clamping mechanism; and controlling the electric motor, based on themonitored parameter, to limit the clamping force.
 16. The method ofclaim 15, wherein the electric motor is controlled to prevent operationof the electric motor beyond a predetermined maximum limit based onpredetermined properties of the tissue to be clamped.
 17. The method ofclaim 16, wherein the predetermined maximum limit is selected to limittrauma to the clamped tissue to a predetermined acceptable level. 18.The method of claim 16, wherein the predetermined maximum limit is belowa predetermined deflection limit of the clamping mechanism.
 19. Themethod of claim 15, further comprising adjusting a driving speed of theelectric motor to limit the clamping force.